1. Field of Invention
This invention relates to an improved air-to-ground communications system for airline passengers that interfaces with landline telephone networks. More particularly, the invention concerns an air-to-ground system serving many aircraft, where each aircraft is matched with an optimum one of many base stations in a network, and wherein communications channels are dynamically reallocated between base stations on a system managed basis to most efficiently use the available radio spectrum.
2. Description of Related Art
A currently existing system, operated by the Assignee of the present application, uses a network of ground stations to provide telecommunications coverage for a plurality of aircraft. Each ground station conducts communications with passengers on aircraft within its range, on specific FCC allocated radio frequency channels, using one or more pilot channels and up to 31 additional voice channels that accompany each pilot channel. The pilot channels carry information necessary for an aircraft to select an optimal base station as determined by its position and flight direction, and subsequently to utilize unoccupied voice channels associated with that base station. Following base station selection, the voice channels carry encoded conversations between the aircraft, through the selected base station, to called numbers on landline networks.
U.S Pat. No. 4,419,766, entitled "Method and Means for Providing Improved Air/Ground Radio Telephone Communications" relates to such a system. Likewise, U. S. Pat. No. 3,952,251, entitled "Narrow Bandwidth, Compatible Single SideBand (CCSB) Transmission System, and Three Tone Generator Used Therein" describes a transmission technique having utility in such a system.
However, additional needs inevitably accompany installation and use of such a system. At present, base stations established to cover geographic regions experiencing light air traffic require assignment of all 31 available voice channels, by definition, despite a possible lack of need of these channels, and despite an unsatisfied need for channels in adjacent high traffic areas.
The current system also could be improved where an aircraft selects an optimal pilot channel but finds that all 31 designated voice channels for that pilot channel are busy. Currently, the aircraft might then select and establish contact with a less desirable base station, thereby providing less desirable voice quality calls and less predictable call duration.
Also, technical limitations currently restrict coverage in each geographical zone to a maximum of three pilot channels. Because of this, the maximum load of each base station is 93 voice channels, or in other words, only 93 conversations at any time. Increasing air-to-ground communications traffic could easily outpace this channel availability.
In addition, an improved system is needed to better facilitate calls made from aircraft while on the ground at airports. The present system handles a substantial number of passenger calls made from aircraft at airports while the aircraft are delayed, stopping between flights, departing late, or arriving late. As a result, the present system inefficiently allocates a limited number of voice channels to each geographic region despite the fact that many calls are placed, conducted, and ended within the confines of an airport. Because of the resulting hardship placed on airborne callers that receive busy signals due to a high concentration of system calls conducted at airports, an improved system to solve this problem is necessary.
The relative altitude of aircraft affects potential interference between calls. A single frequency channel may be simultaneously used at different locations so long as the locations are sufficiently spaced apart to prevent interference. However, as the altitude of an aircraft increases, the amount of surface area of the earth over which its on-board transceivers can communicate increases. Thus, higher flying aircraft experience a greater potential for interference between frequency channels simultaneously being used at diverse surface locations. The design of the current frequency spectrum allocation methodology assumes that all aircraft are at a predetermined "maximum" altitude to prevent channel interference. This forces an inefficient use of channels for aircraft flying at lower altitudes. In addition, aircraft occasionally exceed the predetermined "maximum" altitude. When this happens, channel interference results. An improved system is needed for better spectrum efficiency. Moreover, as aircraft technology advances to permit increased aircraft altitudes, improvements are needed to prevent such increased altitudes from causing increased interference.
Likewise an improved system is needed to accommodate the "moving peak" phenomenon. Call demand generated by airplanes geographically shifts as flights progress. One factor to which this condition is attributable is the regular departure, on competing routes, of waves comprising different airline flights between city pairs. At the beginning of each wave there is moderate demand placed upon the base stations that cover each of the cities in the city pair. But, as the aircraft depart at regular time intervals from one city to another, the peak call demand moves toward a point between the cities. At the midpoint base station, the call demand is effectively doubled as the aircraft pass each other.
In addition, airline service in, for example, the United States generally involves a primary east to west wave of aircraft each morning and a subsequent west to east return wave each afternoon. North-south flights throughout the day cross this pattern and generate complementary load factors on the air-to-ground communications system. The effect of this entire traffic pattern can be analogized to a series of multi-directional waves moving across a body of water. At certain points, the waves intersect, join, and become critically larger. Because of such a "moving peak" effect, improvement in the current system will be needed to manage peak call demand as flight traffic and call traffic increases.
Finally, an improved system is needed to solve recurring irregularities in air-to-ground call traffic. Generally call volume in both air-to-ground and landline telephone use is highly predictable. Usually, air-to-ground call volume for a particular geographic region can be reliably predicted by using data such as the time of the year, time of day, and flight patterns near that region. However, unique circumstances such as flight delays, adverse weather conditions, and other factors can cause air-to-ground call volume to shift unexpectedly in affected regions. Because of such occurrences, means are needed to record the effects of these problem factors, provide solutions to be used if the same problem factors arise again, and apply the solutions as needed in the future.